swap on zvol causes high system latency when memory pressure occurs

[codyps]

System information

Type	Version/Name
Distribution Name	arch
Distribution Version	2019-10-09
Linux Kernel	5.3.1-arch1-1-ARCH
Architecture	x86_64
ZFS Version	zfs-0.8.2-1

Describe the problem you're observing

When using a zvol as swap, if something pushes ram usage higher (for example, starting a virtual machine), then high system latency (visible in Xorg) occurs: display stops updating, mouse stops moving, keyboard stops responding. Latency in ssh sessions appears minimal. When the swap device is removed (swapoff), the system behavior returns to normal (even with the memory consuming application still running).

Describe how to reproduce the problem

zfs create -V 4G -b $(getconf PAGESIZE) -o compression=zle -o logbias=throughput -o sync=always -o primarycache=metadata -o secondarycache=none -o com.sun:auto-snapshot=false ${ZFS}/swap
mkswap /dev/zvol/${ZFS}/swap
swapon /dev/zvol/${ZFS}/swap
# perform enough zfs access & run applications so that memory is almost filled
# start something that allocates enough memory so that zfs would need to flush some pages from the ARC (this is a presumption given the behavior. I'm starting VirtualBox vms with 8GiB of ram allocated to them)

Include any warning/errors/backtraces from the system logs

No warnings/errors/etc in system logs from this timeframe

[devZer0]

#7734

[stale]

This issue has been automatically marked as "stale" because it has not had any activity for a while. It will be closed in 90 days if no further activity occurs. Thank you for your contributions.